Solar panels typically have a junction box, which is sometimes also referred to as a PV (photovoltaic) junction box, a (PV) junction and connecting box, or a (PV) connecting box. This junction box has a typically flat dielectric housing and is mounted, e.g. glued, to the back side of the solar panel oriented away from the sun. The solar panels have voltage-carrying conductors, typically in the form of thin conductor bands referred to as so-called ribbons, which extend out from the back of the solar panels and constitute the electrical poles of the solar panel, i.e. between which the voltage generated by the solar panel is present. A solar panel typically has a plurality of solar cells connected in series and/or in parallel in the solar panel. Optionally, one or more intermediate takeoffs between the solar cells or groups of solar cells of the solar panel are routed out from the solar panel and are contacted in the junction box. The interior of the junction box for the solar panel usually includes input-side connecting elements or terminals—e.g., spring-loaded terminals that are used to connect and contact the conductor ribbons of the positive pole and negative pole of the solar panel, which protrude through the bottom of the junction box into the interior of the junction box housing—and a bypass diode for the solar panel and optionally, further connecting elements and further bypass diodes in solar panels with one or more intermediate takeoffs.
The junction box also has output-side connecting elements or terminals, e.g. in the form of spring-loaded terminals or plug connectors, for connecting external connecting lines so that by means of the output-side connecting elements and the input-side connecting elements that are connected to them inside the junction box, the first and second external connecting lines can be electrically connected to the positive pole and negative pole of the solar panel in order to wire the solar panels.
Examples of such junction boxes for solar panels are described in DE 10 2007 037 130 and DE 10 2007 042 547.
A solar generator is typically composed of a plurality of solar panels, which are in turn connected in series to form so-called strings in order to achieve a DC solar generator voltage that can be as high as 1,000 V in modern systems. Furthermore, depending on the number of interconnected solar panels and their individual voltages, a plurality of strings may in turn be connected in parallel. Because of the high voltage in the DC voltage part of the solar plant and the currents that are produced, which can be in the ampere range, when hazardous incidents such as a fire occur, there is an acute risk of rescue personnel being exposed to potentially lethal voltages. In particular, firefighters attempting to extinguish a fire burning on a roof structure equipped with a solar plant would risk exposure to potentially lethal voltages conducted by the fire-fighting water. This hazard is particularly present during the daytime, but even at night, the residual light can still result in dangerous voltages in large systems. Hazards of this kind, however, are present not only in the event of a fire, but can also be present with the occurrence of other unusual hazardous incidents or accidents, storm damage or motor vehicle collisions with free-field systems, to name but a few examples while making no claim as to the completeness of this list.
A solar plant does typically have a main circuit breaker or main disconnect switch in the so-called (string) combiner box, but in the event of damage to the solar panels or the connecting lines due to fire, water, hail, etc., this cannot be used to disconnect the region upstream of the (string) combiner box. Moreover, it is also not possible to selectively disconnect individual solar panels.
Furthermore, solar panels produce voltage any time they are exposed to light, which can be the case even during the installation of solar junction boxes to the solar panel, storage, transport, and in particular, installation of solar panels at the destination point. For this reason, even during the installation of solar panels and the connection of connecting lines, when there is corresponding exposure to light, high voltages can be produced that require safe handling and special touch-proofing. Even a single solar panel, due to a series connection of a plurality of solar cells in the solar panel, can produce a voltage that requires safety precautions. For this reason, touch-proof plug connectors are typically used, e.g., from the SUNCLIX® system produced by the applicant, which ensure a high degree of safety. However, it can nevertheless be desirable—in order to further increase safety—to switch the solar generator so that it is as voltage-free as possible, in fact connecting it upstream of the main circuit breaker, in particular directly at the solar panels.
DE 10 2009 024 516 A1 has disclosed a safety device for a junction box, which is for producing an electrical connection to a solar panel and produces an electrically conductive cross connection between the connecting elements so that in the active state, the output voltage is reduced to zero.
When the cross connection is produced, however, a power loss occurs in the safety device, which can possibly lead to undesirable heating. Remaining in this state for a long time can, under certain circumstances, result in damage to the safety device or to the solar panels.
In addition, the safety device is primarily conceived to produce a short circuit in the event of a fire, e.g., a hazardous incident that occurs during ongoing operation. Bringing the safety device into the state in which the output voltage is reduced to zero requires an active control intervention for this purpose. In the event of a hazardous incident such as a fire, under certain circumstances, the safety device may itself already be damaged or destroyed by the time the arriving firefighters, for example, wish to activate it, so that at this point, it may no longer be possible to activate the safety device and the desired safety functionality may no longer be effective.
Consequently, although various safety measures for photovoltaic generators do already exist, they are nevertheless in need of improvement.